1. Technical Field
The present invention relates to a position calculating method and a position calculating device.
2. Related Art
As a position calculating system employing a positioning signal, a GPS (Global Positioning System) is widely known. The GPS is used for position calculating devices incorporated in a cellular phone and a car navigation apparatus. The GPS performs position calculating operation for calculating a three-dimensional coordinate value, which indicates the position of an apparatus in which the GPS is incorporated, and a clock error on the basis of information such as the positions of plural GPS satellites and pseudo distances from the GPS satellites to the apparatus.
In the position calculation, first, the GPS calculates satellite information such as the positions, speeds, and moving directions of the GPS satellites on the basis of navigational data such as almanac and ephemeris superimposed on GPS satellite signals originated from the GPS satellites. Although the almanac can be an effective clue in acquiring a satellite, in general, the almanac is not used for the position calculating operation because accuracy of a satellite orbit is low. On the other hand, the ephemeris not only can be an effective clue in acquiring a satellite but also can be used for the position calculating operation because accuracy of a satellite orbit is high. Therefore, for example, when the GPS starts position calculation without holding the ephemeris, the GPS has to acquire the ephemeris from the GPS satellite signal. As a result, first position calculating time (TTFF: Time to First Fix) increases.
The ephemeris superimposed on the GPS satellite signal includes a value of a parameter called URA Index as an index value that indicates reliability of a satellite orbit. As the URA Index is smaller, reliability of satellite information as the ephemeris is higher. Therefore, the smaller URA Index is more suitable for position calculation. JP-A-2003-279637 discloses a technique for determining satellites used for the position calculation on the basis of the URA Index included in the ephemeris.
In a technique devised in recent years, an information providing apparatus such as a server generates ephemeris effective for a long period such as one week (hereinafter referred to as “long-term predicted ephemeris (long-term predicted orbit data)”) and performs position calculation using the ephemeris rather than performing the position calculation using ephemeris transmitted from a GPS satellite.
As one of methods of defining the long-term predicted ephemeris, a method of defining the long-term predicted ephemeris in a data format same as that in normal ephemeris is conceivable. Specifically, this is a method of approximating a satellite orbit using the Kepler's elliptical orbit model, which is one of approximation models of a satellite orbit, and defining the long-term predicted ephemeris according to a value of a parameter of a model formula (hereinafter referred to as “satellite orbit parameter”) at that point. A satellite prediction calendar (predicted position data) including predicted positions obtained by predicting future positions of a positioning satellite in time series at an interval of predetermined time is provided from a predetermined commercial system. The approximation calculation by the Kepler's elliptical orbit model can be performed by using the satellite prediction calendar.
However, it is found that the predicted positions of the positioning satellite included in the satellite prediction calendar tend to deviate from actual positions of the positioning satellite further in the future. Therefore, when the approximation calculation by the Kepler's elliptical orbit model is performed to generate long-term predicted ephemeris, it is likely that a satellite orbit calculated by the approximation calculation deviates from an actual satellite orbit further in the future from generation date and time. Therefore, in some case, depending on a point when the position calculating apparatus performs the position calculation, the position calculating apparatus performs the position calculation using long-term predicted ephemeris having low reliability that deviates from the actual satellite orbit. This causes deterioration in accuracy of the position calculation.